1. Field of the Invention
The present invention relates to differential pressure flow instruments for determining the rate of flow of a fluid through a pipe and more specifically to orifice fittings.
2. Description of the Prior Art
The predominant measurement in the process industry is flow rate. The need for accuracy in the measurement of fluid flow in the process industry is extremely important. A small error in the measurement of large quantities of a fluid through a pipe translates into a loss of income.
Normally, wherever possible, flow rates are measured with a flat plate orifice which is installed in the pipe carrying the fluid. Oftentimes, the flat plate orifice is housed in an orifice flow meter called an orifice fitting as shown in FIG. 1. The flat plate orifice has a circular bore whose diameter is less than the diameter of the pipe in which it is installed. As Venturi discovered in the late 1700's, fluids gain speed and lose pressure when flowing through a converging section of pipe. Thus, as the fluid converges and accelerates to flow through the smaller bore, it loses pressure. The loss of pressure becomes the differential pressure used to measure the rate of flow.
Many factors affect the accuracy of the readings and the determination of the rate of flow. Standards have been established and adopted by various professional associations. The most popular basis for orifice flow meter design and installation and the accepted standard of orifice flow meter design and installation for the natural gas industry and the process industries in the United States and Canada is the American Gas Association (AGA) publication, Orifice Measurement of Natural Gas, Gas Measurement Committee Report No. 3. The American Petroleum Institute (API) incorporates the AGA publication in its Manual of Petroleum Measurement Standards, Chapter 14--Natural Gas Fluids Measurement. Among the many standards established in these publications is the maximum allowable orifice plate bore eccentricity .SIGMA.. The orifice plate bore must be concentric with both the upstream and downstream inside wall of the orifice plate holder, although the eccentricity with respect to the upstream side is the most critical.
FIG. 1 shows a typical orifice fitting, generally designated as F, representative of a class of orifice holders that is widely used throughout the industry. It is also noted that FIG. 1 also shows the present invention generally designated as 100. Referring to FIG. 1, the bore 12 of the orifice plate 10 must be concentric or within an allowable eccentricity .SIGMA. with the upstream and downstream inside diameter of the bore of the orifice fitting. The orifice plate is held in place in a carrier plate by a flexible, pliant seal ring. Due to the pressures and temperatures experienced by the orifice plate 10 and the seal ring under operating conditions, the orifice plate 10 may move radially a distance of 0.005 inches to even 0.015 inches, thus possibly exceeding the allowable eccentricity of the orifice plate bore with respect to the bore of the orifice fitting.
It is desirable to have an economical, simple and extremely dependable apparatus for ensuring that the orifice plate bore remains concentric or within the allowable eccentricity in the orifice fitting under operating conditions.